Are these pistons trashed?

[jrs_diesel]

Re: Are these pistons trashed?

Only tidbit left to add, as everyone else covered the rest. When you sand and polish, try not to make any vertical sanding strokes. You'll want to try and replicate crosshatching as best as possible. You can start off by sanding horizontally and then make the last few passes in a crosshatch pattern, in order to speed things up.

Any pics of the cylinders themselves? I'd be curious what they look like. Maybe a quick honing may be in order to remove any ridges. For practice, you may wish to buy a crappy lawn mower engine off CL or a nearby small engine place and sand and hone that first. Old lawn-boys make good practice, since they suck to begin with.

EDIT: After you finish with the pistons, you may wish to check this guy's page: http://www.sschapterpsa.com/ramblings/OMC_6hp.htm

He goes part by part through your exact motor

That website has been a great help, right up there with my OEM service manual. Thanks for the advice, I am going to try and salvage these. Here are the pictures of the cylinder walls.

http://forums.iboats.com/showthread.php?t=378211

jrs diesel,

Soda blasting the pistons will probably be the least "invasive" medium to use to clean-up the ring grooves.

As far as the lower seal falling apart, there's your answer as to why the lower cylinder was worse than the top piston.

New pistons (for an engine from that era) will be hard to get or at a minmum cost prohibitive. Soda blast them. Cross-hatch the pistons (staying away from the intake side skirt area) install with new rings. As far as taking off too much material, I wouldn't worry about that. If the pistons are cast, material comes off quicker than if they're forged. Forged pistons have a machined appearance in the interior of the piston. A cast piston should have a P/N cast inside and no evidence of machining. In either case, I think you'll be fine.

Before going any further, measure the cylinder walls. Are the pistons over-sized now, or a standard bore? How much piston skirt clearance? If you have .006"~ .008" piston skirt clearance but new pistons aren't available, there isn't much you can do anyway.

Use a "berry hone" to cross-hatch the cylinder walls. Sand a cross-hatch onto the pistons, assemble with new rings and I think you'll be surprised as to how well the engine will run.

My guess would be that the lower seal failed from heat and or long periods of improper storage. Seals will actually weld themselves to the crankshaft. After sitting for an extended period, the first time you try to start, the seal rips. Then, that clyinder runs lean, and you've got a scoured piston. It has nothing to do with how little the engine was used. Instead, how long or often it was stored.

My next guess would be the crank and con rod bearings are toast.

Hope this helps give you the confidence of using the old pistons.

I can get pistons from BRP, but not cheaply. $52 each, plus $13 per ring set. Haven't found anyone else that sells them.

The cylinder walls measured 1.935, and the skirts at 1.933. So by your figures I should be alright?

I think the lower seal had failed from its previous owner. This motor was a saltwater sailboat motor, that wasn't run very long, or often. It does not appear that the previous guy flushed it much (or at all). Plus the spring had rusted itself in half.

These pistons were indeed cast, and have P/N 303338 on both of them.

The crankshaft and connecting rod bearings looked fine and smooth. All 60 needles were fine No damage that I could see. I did have to press the wrist pins out of the pistons though. The hammer and punch method that the book suggests did not budge either one.

 

[kenmyfam]

Re: Are these pistons trashed?

They be toast !!!!

 

[1946Zephyr]

Re: Are these pistons trashed?

There is no way in Hell I would re-use those pistons, especially if you pawn this off as a "rebuilt engine" and someone sees the poor workmanship here. It just don't look good re-installing scored up pistons.

 

[kahuner]

Re: Are these pistons trashed?

Geez; I had I known that you're only talking about $53 per piston plus ring cost, I would've said replace the pistons!!!!! I thought you were facing pistons not available at all or; $188 ea!!

For $132............replace the pistons and rings.

 

[jrs_diesel]

Re: Are these pistons trashed?

There is no way in Hell I would re-use those pistons, especially if you pawn this off as a "rebuilt engine" and someone sees the poor workmanship here. It just don't look good re-installing scored up pistons.

Good point. I don't plan on selling this motor for quite a while. Since I am rebuilding it, I want to get as much use out of my efforts.

I will be buying a set of pistons. I'll use the old ones as paperweights.